How to open a stuck flange without stripping - tamron 26A

[bghomofaber]

I have found at a bargain price another tamron 26a, alas last lens group is dirty (or has some delamination). having already one with good lenses but a semi stuck barrel, I tried to transplant the last lens group of the latter into the former.

However he flange keeping the last lens group seems stuck.

My caliper cannot open the flange, it keeps sliding out , as the slot is both shallow and very narrow, too narrow for my caliper prongs that are unable to firmly establish themselves within the slot.

I also feel the flange to be blocked somehow.

Any suggestion?